JP4240275B2 - Nonaqueous electrolyte secondary battery - Google Patents
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- JP4240275B2 JP4240275B2 JP2002171021A JP2002171021A JP4240275B2 JP 4240275 B2 JP4240275 B2 JP 4240275B2 JP 2002171021 A JP2002171021 A JP 2002171021A JP 2002171021 A JP2002171021 A JP 2002171021A JP 4240275 B2 JP4240275 B2 JP 4240275B2
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Description
【０００１】
【発明の属する技術分野】
本発明は、非水電解質二次電池に関する。
【０００２】
【従来の技術】
近年、携帯電話やノート型パーソナルコンピュータ等の携帯機器の普及に伴い、小型化、高容量化が可能な電池の開発が進められている。なかでも、リチウムイオン電池などの非水電解質二次電池は、高い作動電圧、高いエネルギー密度を有することから、このような要請に応え得るものとして期待されている。
【０００３】
リチウムイオン二次電池の正極には、リチウムイオンの吸蔵及び放出が可能なリチウム含有金属複合酸化物が正極活物質として用いられている。このリチウム含有金属複合酸化物は、一般に、粒子状に調製され、導電性を向上させるためにカーボンブラック等の導電剤が添加されて正極合剤として用いられるている。そして、導電剤は一般に、正極合剤１００重量部に対して１〜１０重量部程度添加されている。
【０００４】
【発明が解決しようとする課題】
導電剤として用いられているカーボンブラックは、微粒子が連なった鎖状構造を備え、黒鉛などの他の炭素質材料と比較して大きな表面積を有する。これにより、多くの正極活物質との接触が可能となって正極合剤の導電性が向上するから、非水電解質二次電池の正極合剤に添加する導電剤として、しばしば用いられている。しかしながら、カーボンブラックを正極合剤の導電剤として用いても、高率放電特性は必ずしも十分ではなかった。
【０００５】
本発明は上記のような事情に基づいて完成されたものであって、高率放電特性の良好な非水電解質二次電池を提供することを目的とする。
【０００６】
【課題を解決するための手段】
本発明者らが、上記の課題に鑑みて鋭意検討を行った結果、高率放電特性が向上しない理由は以下のように考えられた。
【０００７】
カーボンブラックは、黒鉛などの他の炭素質材料より表面積が大きいため、正極活物質との接触が良好であり、導電剤として用いると正極合剤の導電性が良好となる。ところが、カーボンブラックは、嵩密度が小さく、溶媒を吸収しやすい性質も備えている。よって、カーボンブラックを導電剤として正極合剤に添加し、溶媒とともに混練すると、溶媒を吸収して凝集するため、正極合剤中では、カーボンブラックが均一に分散されないものと考えられる。従って、合剤中にカーボンブラックがほとんど存在せず、導電性が向上していない部分があるため、高率放電特性が不十分になるものと推測された。
【０００８】
このような推測のもと、本発明者らが検討したところ、正極合剤に導電剤としてカーボンブラックのみならず、鱗片状黒鉛も含有させることにより、高率放電特性を改善できることを見出した。
【０００９】
そして、本発明者らが繰り返し実験した結果によると、鱗片状黒鉛とカーボンブラックとの重量の和に対する鱗片状黒鉛の重量の割合を０重量％より大きく、１５重量％未満とすることにより、高率放電特性を改善できることが判明したのである。
【００１０】
鱗片状黒鉛は、カーボンブラックと比較して溶媒を吸収しにくい性質を備えている。従って、鱗片状黒鉛を導電剤として正極合剤に添加すると、導電剤による溶媒の吸収が減少して、導電剤が凝集しにくくなる。このため、導電剤が正極合剤中で均一に分散されるから、高率放電特性が向上するものと考えられる。
【００１１】
一方、鱗片状黒鉛とカーボンブラックとの重量の和に対する鱗片状黒鉛の重量比率が、１５重量％以上の場合には、導電剤中のカーボンブラックの割合が低下するために、正極活物質と導電剤との接触が不十分となる。その結果、正極合剤層の導電性も不十分となり、高率放電特性がそれほど向上しないものと考えられる。
【００１２】
さらに、高率放電特性と正極活物質の表面積との関係についても検討を行ったところ、正極活物質の表面積を２．５ｍ２／ｇ以下とすることにより、高率放電特性をさらに向上できることが判明した。その理由は以下のように推測される。
【００１３】
正極合剤には、正極活物質及び導電剤とともに、これらを固着させるための結着剤を、溶液あるいはディスパージョンの状態で添加している。このため、正極活物質の表面積が大きい場合には、結着剤を溶解等している溶媒が正極活物質に吸収されてしまい、正極合剤中への結着剤の分散が困難となると考えられる。
【００１４】
従って、正極活物質の表面積を２．５ｍ２／ｇ以下とすることにより、結着剤を溶解している溶媒等が正極活物質に吸収されにくくなるから、正極合剤へ容易に分散させることができる。このため、正極合剤層のひび割れが生じにくくなり、このひび割れにより正極合剤層の電気伝導が妨害されないから、高率放電特性がさらに改善できるものと考えられる。
【００１５】
本発明は、以上の知見に基づいてなされたものである。
即ち、請求項１の発明は、正極合剤を含有する正極と、負極と、非水電解質とを備えた非水電解質二次電池において、前記正極合剤は、正極活物質を含有するとともに、前記正極活物質１００重量部に対して導電剤を１重量部以上１０重量部以下含有し、前記導電剤は、鱗片状黒鉛とカーボンブラックとを含有するとともに、前記鱗片状黒鉛と前記カーボンブラックとの重量の和に対する前記鱗片状黒鉛の重量の割合が１５重量％未満であり、かつ、前記正極活物質の表面積が２．５ｍ ２ ／ｇ以下であることを特徴とする非水電解質二次電池である。
【００１６】
【発明の実施の形態】
本発明の電池に用いられる正極は、正極集電体に正極合剤の層を形成して作製されている。そして、正極合剤には、正極活物質、導電剤、及び結着剤が含有されている。
【００１７】
正極合剤における導電剤の含有量は、正極活物質１００重量部に対して１重量部以上１０重量部以下であり、好ましくは１．５重量部以上８重量部以下、より好ましくは２重量部以上５重量部以下である。導電剤の添加量が１重量部未満であれば正極合剤中での導電剤の割合が不足して、高率放電特性を向上できないためであり、１０重量部より大きければ、正極活物質の割合が減少するため放電容量が不十分となるためである。
【００１８】
正極合剤における導電剤は、鱗片状黒鉛とカーボンブラックとを含有し、鱗片状黒鉛とカーボンブラックとの重量の和に対する鱗片状黒鉛の重量の割合は、０重量％より大きく１５重量％未満であることを要し、好ましくは５重量％以上１３重量％以下、さらに好ましくは７重量％以上１１重量％以下である。鱗片状黒鉛が上記の割合で含有されていると、導電剤を正極合剤に混合する際に凝集しにくくなって、導電剤の分散性が向上し、高率放電特性が向上するためである。
【００１９】
カーボンブラックとしては、特に限定されず、例えば、アセチレンブラック、ケッチェンブラック、チャンネルブラック、ファーネスブラックを好ましく用いることができる。カーボンブラックは、ガス状、あるいは霧滴状とした炭化水素を不完全燃焼、または熱分解させることにより製造でき、その原料としてはアセチレン、石油、石炭等を使用することができる。カーボンブラックは、表面積が２５ｍ２／ｇ以上のものが好ましく、５０ｍ２／ｇがさらに好ましい。正極活物質と導電剤との接触が良好となり、導電性が向上するためである。
【００２０】
鱗片状黒鉛としては、天然黒鉛、または人造黒鉛を用いることができ、結晶性が高く、安価なため天然黒鉛が好ましい。人造黒鉛は、石炭、又はピッチなどの有機材料を炭化し、２０００℃以上で黒鉛化して製造することができる。
鱗片状黒鉛は、Ｘ線回折法で得られる（００２）面間隔が、０．３３６０ｎｍ以下のものが好ましく、（００２）面のＣ軸結晶厚みが、１００ｎｍ以下であることが好ましい。鱗片状黒鉛の結晶性が高く、電子伝導性が良好となるためである。鱗片状黒鉛のレーザー回折法による累積５０％粒径は８０μｍ以下のものが好ましい。正極活物質及びカーボンブラックとの接触が良好となり、高率放電特性が向上するためである。
【００２１】
正極合剤には、導電剤として、炭素繊維等の他の導電剤が含有されていてもよい。正極集電体としては、例えば、アルミニウム箔、銅箔、ステンレス箔、ニッケル箔等を用いることができる。
【００２２】
正極活物質はリチウムを吸蔵及び放出可能な材料であれば特に限定されず、例えば、ＬｉＣｏＯ２、ＬｉＮｉＯ２、ＬｉＮｉ１／２Ｍｎ１／２Ｏ２、ＬｉＮｉ１／３Ｍｎ１／３Ｃｏ１／３Ｏ２、ＬｉＣｏｘＮｉ１−ｘＯ２、ＬｉＭｎ２Ｏ４、Ｌｉ２Ｍｎ２Ｏ４、ＭｎＯ２、ＦｅＯ２、Ｖ２Ｏ５、Ｖ６Ｏ１３、ＴｉＯ２またはＴｉＳ２等を用いることができる。これらの材料は、１種類を単独で、あるいは２種以上を混合して用いることができる。
【００２３】
正極活物質の表面積は、２．５ｍ２／ｇ以下が好ましく、より好ましくは２．０ｍ２／ｇ以下、さらに好ましくは１．５ｍ２／ｇ以下である。正極活物質の表面積を上記範囲とすることにより、正極合剤中での結着剤の分散が良好となるためである。なお、正極活物質の表面積はＢＥＴ法により求めることができる。
【００２４】
結着剤としては、特に限定されず、例えば、ポリフッ化ビニリデン、ポリテトラフルオロエチレン、フッ素ゴム、スチレンブタジエンゴム等を利用できる。その形態としては、例えば、Ｎ−メチル−２−ピロリドン等の有機溶媒溶液、水性ディスパージョン等として用いることができる。
【００２５】
本発明の電池に用いられる負極には、負極活物質が含有されている。負極は、例えば、負極集電体に負極活物質を含有する負極合剤の層を形成することによって作製することができる。負極活物質としてはリチウムイオンを吸蔵・放出可能な物質であれば特に限定されることなく用いることができる。リチウムイオンを吸蔵・放出可能な物質としては、例えば、炭素質材料、金属酸化物、金属リチウム等を用いることができる。炭素質材料としては、例えば公知のコークス類、ガラス状炭素類、グラファイト類、難黒鉛化性炭素類、熱分解炭素類、炭素繊維を用いることができ、金属酸化物としては、Ｎｂ２Ｏ５、Ｌｉ４／３Ｔｉ５／３Ｏ４、ＳｎＢｘＰｙＯｚ（ｘ＝０．４〜０．６、ｙ＝０．６〜０．４、ｚ＝（２＋３ｘ＋５ｙ）／２）、ＳｉＯ等を用いることができる。負極集電体としては、銅箔、ステンレス箔などを用いることができる。
【００２６】
本発明の電池に用いられる非水電解質としては、非水電解液、固体電解質を用いることができる。非水電解液は、非水溶媒に電解質塩を溶解してなり、非水溶媒は電池内の酸化還元電位に耐えうるものであれば特に限定されない。非水溶媒としては、例えば、エチレンカーボネート、プロピレンカーボネート等の環状カーボネート、ジエチルカーボネート、ジメチルカーボネート、エチルメチルカーボネート等の鎖状カーボネート、γ−ブチロラクトン等の環状エステル等を使用することができる。
【００２７】
電解質塩としては、非水電解質二次電池に通常使用される電解質塩であれば特に制限はなく、例えばＬｉＰＦ６、ＬｉＣｌＯ４、ＬｉＢＦ４、ＬｉＡｓＦ６、ＬｉＰＦ３（Ｃ２Ｆ５）３、ＬｉＣＦ３ＣＯ２、ＬｉＣＦ３（ＣＦ３）３、 ＬｉＣＦ３（Ｃ３Ｆ５）３、ＬｉＣＦ３ＳＯ３、ＬｉＮ（ＳＯ３ＣＦ３）３、ＬｉＮ（ＳＯ３ＣＦ３ＣＦ３）３、ＬｉＮ（ＣＯＣＦ３）およびＬｉＮ（ＣＯＣＦ３ＣＦ３）３などの塩もしくはこれらの混合物を用いることができる。これらの電解質塩濃度は、特に限定されず、０．５〜２．０ｍｏｌ／ｌとすることができる。
固体電解質としては、公知の固体電解質を用いることができ、例えば無機固体電解質、ポリマー固体電解質を用いることができる。
【００２８】
【実施例】
以下、実施例を挙げて本発明をさらに詳細に説明する。
１．非水電解質二次電池の作製
＜実施例１＞
（正極の作製）
まず、正極活物質を次のようにして調製した。炭酸リチウム（Ｌｉ２ＣＯ３）０．５ｍｏｌと炭酸コバルト（ＣｏＣＯ３）１ｍｏｌとを混合し、この混合物を空気中８５０℃で２０時間加熱処理して組成式ＬｉＣｏＯ２で表されるリチウムコバルト複合酸化物を得た。このリチウムコバルト複合酸化物を乳鉢で１時間粉砕し、ＢＥＴ法による表面積２ｍ２／ｇのリチウムコバルト複合酸化物粉末を得た。なお、表面積の測定には、島津製作所製マイクロメリテックス、ジェミニ２３７０を使用した。測定は、液体窒素を用いた低温ガス吸着法により行い、ＢＥＴ法で解析した。
【００２９】
正極活物質としてリチウムコバルト複合酸化物粉末（ＬｉＣｏＯ２）を９４重量部、結着剤としてポリフッ化ビニリデンを４重量部、導電剤としてカーボンブラックを１．９０重量部、及び同じく導電剤として鱗片状黒鉛を０．１０重量部、Ｎ−メチルピロリドンと共に混練し、正極合剤ペーストを調製した。正極合剤ペーストを、厚さ２０μｍのアルミニウム箔からなる集電体の両面に均一に塗布、乾燥、プレスし、正極合剤層を形成した。その後、裁断し帯状の正極を作製した。なお、カーボンブラックとして、表面積が６８ｍ２／ｇであるアセチレンブラックを用いた。鱗片状黒鉛は、天然黒鉛を粉砕して得たものを用いた。鱗片状黒鉛は、レーザー回折法による累積５０％粒径が５０μｍ、（００２）面間隔が０．３３６０ｎｍであり、（００２）面のＣ軸結晶厚みが５０μｍのものを用いた。ポリフッ化ビニリデンは、５％のＮ−メチル−２−ピロリドン溶液の状態として８０重量部用いた。
【００３０】
上記導電剤及び結着剤の混合量は、正極合剤１００重量部に対する混合量であり、正極活物質１００重量部に対するそれぞれの混合量は、ポリフッ化ビニリデン４．２６重量部、カーボンブラック２．０２重量部、鱗片状黒鉛０．１１重量部となる。そして、正極活物質１００重量部に対する鱗片状黒鉛の混合量をＧ、カーボンブラックの混合量をＣ、鱗片状黒鉛とカーボンブラックとの重量の和に対する鱗片状黒鉛の重量の割合をＧ×１００／（Ｇ＋Ｃ）として、それぞれを表１に示す。
【００３１】
（負極の作製）
負極は、次のように作製した。負極活物質としての黒鉛９６重量部と、結着剤としてのポリフッ化ビニリデン４重量部とを、溶媒としてのＮ−メチル−２−ピロリドンと共に混練し、負極合剤ペーストを調製した。なお、ポリフッ化ビニリデンは、５％のＮ−メチル−２−ピロリドン溶液として、８０重量部用いた。負極合剤ペーストを、厚さ１０μｍの銅箔からなる集電体の両面に均一に塗布し、正極と同様にして負極を作製した。
【００３２】
（非水電解質の調製）
非水電解質は、エチレンカーボネートとジエチルカーボネートとを、体積比１：１となるように混合し、電解質塩としてのＬｉＰＦ６を、１．０ｍｏｌ／ｌの濃度となるように加えて調製した。
【００３３】
（電池の作製）
セパレータとして、ポリエチレン微多孔膜を使用し、正極、セパレータ、負極、セパレータの順に積層したものを巻回して発電要素を作製した。この発電要素を、角型のアルミニウム製電池ケース内に収納するとともに、電池ケース内に調製した非水電解質を真空注液した。そして、電池ケースを封口し、周知の方法で電池を組み立てた。電池の定格容量は、３６０ｍＡｈとした。
【００３４】
＜実施例２、実施例３、比較例１、及び比較例２＞
正極活物質１００重量部に対する鱗片状黒鉛、及びカーボンブラックの混合量、及び鱗片状黒鉛とカーボンブラックとの重量の和に対する鱗片状黒鉛の重量の割合（Ｇ×１００／（Ｇ＋Ｃ））を表１に示すようにした他は、実施例１と同様にして、実施例２、実施例３、比較例１、及び比較例２の非水電解質二次電池を作製した。
【００３５】
＜実施例４、実施例５、比較例３、及び比較例４＞
正極活物質１００重量部に対する導電剤の混合量を表２に示すようにした他は、実施例２と同様にして、実施例４、実施例５、比較例３、及び比較例４の非水電解質二次電池を作製した。
【００３６】
＜実施例６〜実施例８、参考例１＞
リチウムコバルト複合酸化物を粉砕する時間を変えることにより、表３に示す表面積のリチウムコバルト複合酸化物粉末を得て、このリチウムコバルト複合酸化物正極活物質として用いた他は、実施例２と同様にして、実施例６〜実施例８、および参考例１の非水電解質二次電池を作製した。
【００３７】
２．測定
上記の方法で作製した実施例１〜実施例８、参考例１、及び比較例１〜比較例４の電池について、以下の測定を行った。
【００３８】
（高率放電特性測定）
非水電解質二次電池について、高率放電特性を測定した。充電は、３６０ｍＡ定電流で４．２０Ｖまで、さらに４．２０Ｖ定電圧で、合計３時間行った。一方、放電は３６０ｍＡ定電流で２．７５Ｖまで行った。充放電サイクルは４サイクル行い、第３サイクルの放電容量を低率放電容量とした。さらに、第４サイクルの充電を、３６０ｍＡ定電流で４．２０Ｖまで、さらに４．２０Ｖ定電圧で合計３時間行った後、放電を１０８０ｍＡ定電流で２．７５Ｖまで行って、高率放電容量を測定した。そして、低率放電容量に対する高率放電容量の割合を高率放電容量比とした。このようにして高率放電特性を測定した。
【００３９】
（充放電サイクル特性測定）
充放電サイクル特性を以下のようにして測定した。充電は、３６０ｍＡ定電流で４．２０Ｖまで、さらに４．２０Ｖ定電圧で、合計３時間行った。放電は３６０ｍＡ定電流で行い、終止電圧２．７５Ｖまで行った。第３サイクルの放電容量を初期放電容量とし、初期放電容量に対する第３００サイクルの放電容量の割合を容量保持率とした。
【００４０】
３．結果
（鱗片状黒鉛とカーボンブラックとの割合の検討）
導電剤としての鱗片状黒鉛とカーボンブラックとの重量の和に対する鱗片状黒鉛の重量の割合の異なる電池の、高率放電容量比、及び容量保持率の測定結果を表１に示す。
【００４１】
【表１】
鱗片状黒鉛の重量の割合が、０重量％より大きく１５重量％未満である実施例１〜実施例３の電池の高率放電容量比及び容量保持率は、鱗片状黒鉛が含有されていない比較例１及び１５％以上である比較例２より高い値を示した。
【００４３】
（導電剤の添加量の検討）
導電剤の混合量が異なる電池の、高率放電容量比、及び容量保持率の測定結果を表２に示す。
【００４４】
【表２】
正極活物質１００重量部に対する、導電剤の混合量が１重量部以上１０重量部以下である実施例２、４、５の電池の高率放電容量比及び容量保持率は、導電剤の混合量が１重量部未満である比較例３及び１０重量部を超える比較例４より高い値を示した。
【００４６】
（正極活物質の表面積の検討）
正極活物質の表面積が異なる電池の、高率放電容量比、及び容量保持率の測定結果を表３に示す。
【００４７】
【表３】
正極活物質の表面積が、２．５ｍ２／ｇ以下である実施例２、６〜８の電池の高率放電容量比及び容量保持率は、２．５ｍ２／ｇより大きい参考例１の電池より高い値を示した。
【００４９】
【発明の効果】
本発明によれば、正極合剤に、正極活物質１００重量部に対して導電剤を１重量部以上１０重量部以下含有し、導電剤として鱗片状黒鉛とカーボンブラックとを含有するとともに、鱗片状黒鉛とカーボンブラックとの重量の和に対する鱗片状黒鉛の重量の割合を１５重量％未満とすることにより、非水電解質二次電池の高率放電特性を向上できる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a non-aqueous electrolyte secondary battery.
[0002]
[Prior art]
In recent years, with the widespread use of portable devices such as mobile phones and notebook personal computers, development of batteries that can be reduced in size and increased in capacity has been promoted. Among these, non-aqueous electrolyte secondary batteries such as lithium ion batteries are expected to meet such demands because they have a high operating voltage and a high energy density.
[0003]
As a positive electrode of a lithium ion secondary battery, a lithium-containing metal composite oxide capable of inserting and extracting lithium ions is used as a positive electrode active material. This lithium-containing metal composite oxide is generally prepared in a particulate form, and is used as a positive electrode mixture with a conductive agent such as carbon black added to improve conductivity. The conductive agent is generally added in an amount of about 1 to 10 parts by weight with respect to 100 parts by weight of the positive electrode mixture.
[0004]
[Problems to be solved by the invention]
Carbon black used as a conductive agent has a chain structure in which fine particles are connected, and has a larger surface area than other carbonaceous materials such as graphite. As a result, contact with many positive electrode active materials is possible, and the conductivity of the positive electrode mixture is improved. Therefore, it is often used as a conductive agent added to the positive electrode mixture of the nonaqueous electrolyte secondary battery. However, even when carbon black is used as the conductive agent of the positive electrode mixture, the high rate discharge characteristics are not always sufficient.
[0005]
This invention is completed based on the above situations, Comprising: It aims at providing the nonaqueous electrolyte secondary battery with a favorable high rate discharge characteristic.
[0006]
[Means for Solving the Problems]
As a result of intensive studies by the present inventors in view of the above problems, the reason why the high rate discharge characteristics are not improved is considered as follows.
[0007]
Since carbon black has a larger surface area than other carbonaceous materials such as graphite, the contact with the positive electrode active material is good, and when used as a conductive agent, the conductivity of the positive electrode mixture is good. However, carbon black has a low bulk density and a property of easily absorbing a solvent. Therefore, when carbon black is added to the positive electrode mixture as a conductive agent and kneaded with the solvent, it absorbs the solvent and agglomerates. Therefore, it is considered that the carbon black is not uniformly dispersed in the positive electrode mixture. Therefore, it was speculated that the high-rate discharge characteristics would be insufficient because there was almost no carbon black in the mixture and there was a portion where the conductivity was not improved.
[0008]
Based on such assumptions, the present inventors have examined and found that the high-rate discharge characteristics can be improved by including not only carbon black but also flaky graphite as a conductive agent in the positive electrode mixture.
[0009]
According to the results of repeated experiments by the present inventors, the ratio of the weight of the flaky graphite to the sum of the weights of the flaky graphite and the carbon black is greater than 0% by weight and less than 15% by weight. It has been found that the rate discharge characteristics can be improved.
[0010]
Scale-like graphite has a property that it is difficult to absorb a solvent as compared with carbon black. Therefore, when scaly graphite is added as a conductive agent to the positive electrode mixture, the absorption of the solvent by the conductive agent is reduced, and the conductive agent is less likely to aggregate. For this reason, since the conductive agent is uniformly dispersed in the positive electrode mixture, it is considered that the high rate discharge characteristics are improved.
[0011]
On the other hand, when the weight ratio of the flaky graphite to the sum of the weights of the flaky graphite and the carbon black is 15% by weight or more, the ratio of the carbon black in the conductive agent is lowered, so that the positive electrode active material and the conductive material are electrically conductive. Insufficient contact with the agent. As a result, it is considered that the conductivity of the positive electrode mixture layer becomes insufficient and the high rate discharge characteristics are not improved so much.
[0012]
Furthermore, when the relationship between the high rate discharge characteristics and the surface area of the positive electrode active material was also examined, the high rate discharge characteristics can be further improved by setting the surface area of the positive electrode active material to 2.5 m 2 / g or less. found. The reason is presumed as follows.
[0013]
A positive electrode active material and a conductive agent are added to the positive electrode mixture, and a binder for fixing them is added in the form of a solution or a dispersion. For this reason, when the surface area of the positive electrode active material is large, the solvent dissolving the binder is absorbed by the positive electrode active material, which makes it difficult to disperse the binder in the positive electrode mixture. It is done.
[0014]
Therefore, by setting the surface area of the positive electrode active material to 2.5 m 2 / g or less, the solvent or the like in which the binder is dissolved is hardly absorbed by the positive electrode active material, so that it can be easily dispersed in the positive electrode mixture. Can do. For this reason, the positive electrode mixture layer is unlikely to crack, and the electric conduction of the positive electrode mixture layer is not hindered by the crack, and it is considered that the high rate discharge characteristics can be further improved.
[0015]
The present invention has been made based on the above findings.
That is, the invention of claim 1 is a nonaqueous electrolyte secondary battery comprising a positive electrode containing a positive electrode mixture, a negative electrode, and a nonaqueous electrolyte, wherein the positive electrode mixture contains a positive electrode active material, The conductive agent contains 1 part by weight or more and 10 parts by weight or less of a conductive agent with respect to 100 parts by weight of the positive electrode active material, and the conductive agent contains flaky graphite and carbon black, and the flaky graphite and carbon black. The ratio of the weight of the flaky graphite to the sum of the weight of the non-aqueous electrolyte secondary battery is less than 15% by weight, and the positive electrode active material has a surface area of 2.5 m 2 / g or less. It is.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
The positive electrode used in the battery of the present invention is produced by forming a positive electrode mixture layer on a positive electrode current collector. The positive electrode mixture contains a positive electrode active material, a conductive agent, and a binder.
[0017]
The content of the conductive agent in the positive electrode mixture is 1 to 10 parts by weight, preferably 1.5 to 8 parts by weight, more preferably 2 parts by weight with respect to 100 parts by weight of the positive electrode active material. The amount is 5 parts by weight or less. If the added amount of the conductive agent is less than 1 part by weight, the ratio of the conductive agent in the positive electrode mixture is insufficient, and the high rate discharge characteristics cannot be improved. This is because the discharge capacity becomes insufficient because the ratio decreases.
[0018]
The conductive agent in the positive electrode mixture contains scaly graphite and carbon black, and the ratio of the weight of scaly graphite to the sum of the weights of scaly graphite and carbon black is greater than 0% by weight and less than 15% by weight. In other words, it is preferably 5% by weight to 13% by weight, more preferably 7% by weight to 11% by weight. When the scale-like graphite is contained in the above ratio, it becomes difficult to aggregate when the conductive agent is mixed with the positive electrode mixture, the dispersibility of the conductive agent is improved, and the high rate discharge characteristics are improved. .
[0019]
The carbon black is not particularly limited, and for example, acetylene black, ketjen black, channel black, and furnace black can be preferably used. Carbon black can be produced by incomplete combustion or thermal decomposition of hydrocarbons in the form of gas or mist droplets, and acetylene, petroleum, coal, or the like can be used as a raw material. The carbon black preferably has a surface area of 25 m 2 / g or more, and more preferably 50 m 2 / g. This is because the contact between the positive electrode active material and the conductive agent is improved, and the conductivity is improved.
[0020]
As the flake graphite, natural graphite or artificial graphite can be used, and natural graphite is preferable because it has high crystallinity and is inexpensive. Artificial graphite can be produced by carbonizing an organic material such as coal or pitch and graphitizing at 2000 ° C. or higher.
The scaly graphite preferably has a (002) plane spacing of 0.3360 nm or less obtained by an X-ray diffraction method, and the C-axis crystal thickness of the (002) plane is preferably 100 nm or less. This is because the scaly graphite has high crystallinity and good electron conductivity. The accumulated 50% particle size of the scaly graphite by the laser diffraction method is preferably 80 μm or less. This is because contact with the positive electrode active material and carbon black is improved, and high rate discharge characteristics are improved.
[0021]
The positive electrode mixture may contain other conductive agent such as carbon fiber as the conductive agent. As the positive electrode current collector, for example, aluminum foil, copper foil, stainless steel foil, nickel foil or the like can be used.
[0022]
The positive electrode active material is not particularly limited as long as it is a material capable of inserting and extracting lithium. For example, LiCoO 2 , LiNiO 2 , LiNi 1/2 Mn 1/2 O 2 , LiNi 1/3 Mn 1/3 Co 1 / 3 O 2 , LiCo x Ni 1-x O 2 , LiMn 2 O 4 , Li 2 Mn 2 O 4 , MnO 2 , FeO 2 , V 2 O 5 , V 6 O 13 , TiO 2 or TiS 2 are used. Can do. These materials can be used alone or in combination of two or more.
[0023]
The surface area of the positive electrode active material is preferably 2.5 m 2 / g or less, more preferably 2.0 m 2 / g or less, and still more preferably 1.5 m 2 / g or less. This is because, by setting the surface area of the positive electrode active material in the above range, the dispersion of the binder in the positive electrode mixture is improved. The surface area of the positive electrode active material can be determined by the BET method.
[0024]
The binder is not particularly limited, and for example, polyvinylidene fluoride, polytetrafluoroethylene, fluorine rubber, styrene butadiene rubber, or the like can be used. As its form, for example, it can be used as an organic solvent solution such as N-methyl-2-pyrrolidone, an aqueous dispersion and the like.
[0025]
The negative electrode used in the battery of the present invention contains a negative electrode active material. The negative electrode can be produced, for example, by forming a negative electrode mixture layer containing a negative electrode active material on a negative electrode current collector. As the negative electrode active material, any material that can occlude and release lithium ions can be used without particular limitation. As a substance capable of inserting and extracting lithium ions, for example, carbonaceous materials, metal oxides, metal lithium, and the like can be used. As the carbonaceous material, for example, known cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, carbon fibers can be used, and as the metal oxide, Nb 2 O 5 , Li 4/3 Ti 5/3 O 4 , SnB x P y O z (x = 0.4 to 0.6, y = 0.6 to 0.4, z = (2 + 3x + 5y) / 2), SiO, etc. Can be used. As the negative electrode current collector, a copper foil, a stainless steel foil, or the like can be used.
[0026]
As the non-aqueous electrolyte used in the battery of the present invention, a non-aqueous electrolyte solution or a solid electrolyte can be used. The non-aqueous electrolyte solution is not particularly limited as long as the electrolyte salt is dissolved in a non-aqueous solvent, and the non-aqueous solvent can withstand the oxidation-reduction potential in the battery. As the non-aqueous solvent, for example, cyclic carbonates such as ethylene carbonate and propylene carbonate, chain carbonates such as diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate, cyclic esters such as γ-butyrolactone, and the like can be used.
[0027]
The electrolyte salt is not particularly limited as long as it is an electrolyte salt usually used in a non-aqueous electrolyte secondary battery. For example, LiPF 6 , LiClO 4 , LiBF 4 , LiAsF 6 , LiPF 3 (C 2 F 5 ) 3 , LiCF 3 CO 2 , LiCF 3 (CF 3 ) 3 , LiCF 3 (C 3 F 5 ) 3 , LiCF 3 SO 3 , LiN (SO 3 CF 3 ) 3 , LiN (SO 3 CF 3 CF 3 ) 3 , LiN (COCF 3 ) and salts such as LiN (COCF 3 CF 3 ) 3 or mixtures thereof. These electrolyte salt concentrations are not particularly limited, and can be 0.5 to 2.0 mol / l.
As the solid electrolyte, a known solid electrolyte can be used. For example, an inorganic solid electrolyte or a polymer solid electrolyte can be used.
[0028]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples.
1. Production of Nonaqueous Electrolyte Secondary Battery <Example 1>
(Preparation of positive electrode)
First, a positive electrode active material was prepared as follows. Lithium carbonate (Li 2 CO 3 ) 0.5 mol and cobalt carbonate (CoCO 3 ) 1 mol are mixed, and this mixture is heat-treated in air at 850 ° C. for 20 hours to form a lithium cobalt composite oxide represented by the composition formula LiCoO 2. I got a thing. This lithium cobalt composite oxide was pulverized in a mortar for 1 hour to obtain a lithium cobalt composite oxide powder having a surface area of 2 m 2 / g by BET method. In addition, Shimadzu Corporation micromeritex and Gemini 2370 were used for the measurement of a surface area. The measurement was performed by a low temperature gas adsorption method using liquid nitrogen and analyzed by the BET method.
[0029]
94 parts by weight of lithium cobalt composite oxide powder (LiCoO 2 ) as a positive electrode active material, 4 parts by weight of polyvinylidene fluoride as a binder, 1.90 parts by weight of carbon black as a conductive agent, and also scaly as a conductive agent Graphite was kneaded with 0.10 parts by weight of N-methylpyrrolidone to prepare a positive electrode mixture paste. The positive electrode mixture paste was uniformly applied to both sides of a current collector made of an aluminum foil having a thickness of 20 μm, dried and pressed to form a positive electrode mixture layer. Then, it cut | judged and produced the strip-shaped positive electrode. As carbon black, acetylene black having a surface area of 68 m 2 / g was used. The scaly graphite used was obtained by pulverizing natural graphite. As the flake graphite, a 50% cumulative particle diameter by laser diffraction method was 50 μm, the (002) plane spacing was 0.3360 nm, and the (002) plane C-axis crystal thickness was 50 μm. Polyvinylidene fluoride was used in an amount of 80 parts by weight as a 5% N-methyl-2-pyrrolidone solution.
[0030]
The mixing amount of the conductive agent and the binder is a mixing amount with respect to 100 parts by weight of the positive electrode mixture, and the mixing amount with respect to 100 parts by weight of the positive electrode active material is 4.26 parts by weight of polyvinylidene fluoride, 2. 02 parts by weight and scale-like graphite 0.11 part by weight. And the mixing amount of the flaky graphite with respect to 100 parts by weight of the positive electrode active material is G, the mixing amount of the carbon black is C, and the ratio of the weight of the flaky graphite to the sum of the weights of the flaky graphite and the carbon black is G × 100 / Each is shown as (G + C) in Table 1.
[0031]
(Preparation of negative electrode)
The negative electrode was produced as follows. 96 parts by weight of graphite as a negative electrode active material and 4 parts by weight of polyvinylidene fluoride as a binder were kneaded together with N-methyl-2-pyrrolidone as a solvent to prepare a negative electrode mixture paste. Polyvinylidene fluoride was used in an amount of 80 parts by weight as a 5% N-methyl-2-pyrrolidone solution. The negative electrode mixture paste was uniformly applied to both surfaces of a current collector made of a copper foil having a thickness of 10 μm, and a negative electrode was produced in the same manner as the positive electrode.
[0032]
(Preparation of non-aqueous electrolyte)
The nonaqueous electrolyte was prepared by mixing ethylene carbonate and diethyl carbonate in a volume ratio of 1: 1, and adding LiPF 6 as an electrolyte salt to a concentration of 1.0 mol / l.
[0033]
(Production of battery)
A polyethylene microporous membrane was used as a separator, and a power generation element was produced by winding a laminate of a positive electrode, a separator, a negative electrode, and a separator in that order. The power generation element was housed in a rectangular aluminum battery case, and the nonaqueous electrolyte prepared in the battery case was vacuum injected. And the battery case was sealed and the battery was assembled by the well-known method. The rated capacity of the battery was 360 mAh.
[0034]
<Example 2, Example 3, Comparative Example 1, and Comparative Example 2>
Table 1 shows the amount of flaky graphite and carbon black mixed with 100 parts by weight of the positive electrode active material, and the ratio of the weight of flaky graphite to the sum of the weights of flaky graphite and carbon black (G × 100 / (G + C)). The nonaqueous electrolyte secondary batteries of Example 2, Example 3, Comparative Example 1, and Comparative Example 2 were produced in the same manner as Example 1 except for the above.
[0035]
<Example 4, Example 5, Comparative Example 3, and Comparative Example 4>
The non-aqueous solutions of Example 4, Example 5, Comparative Example 3, and Comparative Example 4 were the same as Example 2 except that the amount of the conductive agent mixed with 100 parts by weight of the positive electrode active material was as shown in Table 2. An electrolyte secondary battery was produced.
[0036]
<Example 6 to Example 8, Reference Example 1 >
The lithium cobalt composite oxide powder having the surface area shown in Table 3 was obtained by changing the time for pulverizing the lithium cobalt composite oxide, and the same procedure as in Example 2 was used except that this lithium cobalt composite oxide positive electrode active material was used. Thus, the nonaqueous electrolyte secondary batteries of Examples 6 to 8 and Reference Example 1 were produced.
[0037]
2. Measurement The following measurements were performed on the batteries of Examples 1 to 8, Reference Example 1, and Comparative Examples 1 to 4 produced by the above method.
[0038]
(High rate discharge characteristics measurement)
The high rate discharge characteristics were measured for the nonaqueous electrolyte secondary battery. Charging was performed at a constant current of 360 mA up to 4.20 V, and further at a constant voltage of 4.20 V for a total of 3 hours. On the other hand, discharging was performed up to 2.75 V at a constant current of 360 mA. The charge / discharge cycle was performed four times, and the discharge capacity of the third cycle was set to a low rate discharge capacity. Further, after charging at the fourth cycle to 4.20 V at a constant current of 360 mA, and further at a constant voltage of 4.20 V for a total of 3 hours, discharging is performed to 2.75 V at a constant current of 1080 mA to increase the high rate discharge capacity. It was measured. And the ratio of the high rate discharge capacity with respect to the low rate discharge capacity was made into the high rate discharge capacity ratio. In this way, high rate discharge characteristics were measured.
[0039]
(Charge / discharge cycle characteristics measurement)
The charge / discharge cycle characteristics were measured as follows. Charging was performed at a constant current of 360 mA up to 4.20 V, and further at a constant voltage of 4.20 V for a total of 3 hours. Discharging was performed at a constant current of 360 mA and was performed up to a final voltage of 2.75V. The discharge capacity of the third cycle was the initial discharge capacity, and the ratio of the discharge capacity of the 300th cycle to the initial discharge capacity was the capacity retention rate.
[0040]
3. Results (examination of the ratio between scaly graphite and carbon black)
Table 1 shows the measurement results of the high-rate discharge capacity ratio and the capacity retention rate of the batteries having different weight ratios of the scale-like graphite to the sum of the weights of the scale-like graphite and the carbon black as the conductive agent.
[0041]
[Table 1]
The high-rate discharge capacity ratio and capacity retention of the batteries of Examples 1 to 3 in which the ratio of the weight of the flake graphite is greater than 0 wt% and less than 15 wt% is a comparison in which no flake graphite is contained. The value was higher than that of Example 1 and Comparative Example 2 of 15% or more.
[0043]
(Examination of the amount of conductive agent added)
Table 2 shows the measurement results of the high-rate discharge capacity ratio and the capacity retention rate of the batteries having different conductive agent mixing amounts.
[0044]
[Table 2]
The high-rate discharge capacity ratio and the capacity retention ratio of the batteries of Examples 2, 4, and 5 in which the mixing amount of the conductive agent is 1 to 10 parts by weight with respect to 100 parts by weight of the positive electrode active material are the mixed amount of the conductive agent. Was higher than those of Comparative Example 3 in which the amount was less than 1 part by weight and Comparative Example 4 in excess of 10 parts by weight.
[0046]
(Examination of surface area of positive electrode active material)
Table 3 shows the measurement results of the high rate discharge capacity ratio and the capacity retention rate of the batteries having different surface areas of the positive electrode active material.
[0047]
[Table 3]
The battery of Reference Example 1 in which the high-rate discharge capacity ratio and the capacity retention ratio of the batteries of Examples 2 and 6 to 8 in which the surface area of the positive electrode active material is 2.5 m 2 / g or less are larger than 2.5 m 2 / g. It showed a higher value.
[0049]
【The invention's effect】
According to the present invention, the positive electrode mixture contains 1 to 10 parts by weight of a conductive agent with respect to 100 parts by weight of the positive electrode active material, and contains flaky graphite and carbon black as the conductive agent. By setting the ratio of the scale-like graphite weight to the sum of the weights of the graphite-like graphite and the carbon black to be less than 15% by weight, the high rate discharge characteristics of the non-aqueous electrolyte secondary battery can be improved.
Claims (1)
前記正極合剤は、正極活物質を含有するとともに、前記正極活物質１００重量部に対して導電剤を１重量部以上１０重量部以下含有し、
前記導電剤は、鱗片状黒鉛とカーボンブラックとを含有するとともに、前記鱗片状黒鉛と前記カーボンブラックとの重量の和に対する前記鱗片状黒鉛の重量の割合が１５重量％未満であり、
かつ、前記正極活物質の表面積が２．５ｍ ２ ／ｇ以下であることを特徴とする非水電解質二次電池。In a non-aqueous electrolyte secondary battery comprising a positive electrode containing a positive electrode mixture, a negative electrode, and a non-aqueous electrolyte,
The positive electrode mixture contains a positive electrode active material, and contains 1 to 10 parts by weight of a conductive agent with respect to 100 parts by weight of the positive electrode active material,
The conductive agent contains scaly graphite and carbon black, and the ratio of the weight of the scaly graphite to the sum of the weights of the scaly graphite and the carbon black is less than 15% by weight.
And the surface area of the said positive electrode active material is 2.5 m < 2 > / g or less, The nonaqueous electrolyte secondary battery characterized by the above-mentioned .
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| JP4527423B2 (en) * | 2004-03-18 | 2010-08-18 | 大日本印刷株式会社 | Coating composition for active material layer, electrode plate for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery |
| KR101491485B1 (en) | 2008-11-18 | 2015-02-11 | 삼성전자주식회사 | Side view type light emitting device and line light source type light emitting device |
| JP5364500B2 (en) * | 2009-08-20 | 2013-12-11 | 古河電池株式会社 | Method for producing positive electrode plate for non-aqueous electrolyte secondary battery |
| EP3026734B1 (en) | 2013-07-24 | 2017-07-19 | Nissan Motor Co., Ltd | Non-aqueous electrolyte secondary battery |
| EP3509144A4 (en) * | 2016-08-31 | 2020-07-15 | Sekisui Chemical Co., Ltd. | Electrode material for electricity storage devices, electrode for electricity storage devices, and electricity storage device |
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